1. Field of the Invention
The present invention relates to a method for purifying (meth)acrylic acid, particularly to a method for purifying a crude (meth)acrylic acid obtained by vapor phase catalytic oxidation, by distillation to obtain highly pure (meth)acrylic acid which is useful for the production of a highly water absorptive resin and for the production of a (meth)acrylic ester. In this specification, (meth)acrylic acid means acrylic acid or methacrylic acid.
2. Discussion of Background
a. As a method for producing (meth)acrylic acid, a method of hydrolyzing the corresponding nitrile compound may, for example, be mentioned. However, at present, a vapor phase catalytic oxidation method of the corresponding hydrocarbon such as propylene or isobutylene, is mainly employed. Recently, a study has been made also on a vapor phase catalytic oxidation method using an inexpensive corresponding alkane as the starting material instead of an olefin.
In the production of (meth)acrylic acid by a vapor phase catalytic oxidation method, firstly the reaction product gas containing (meth)acrylic acid is contacted with an absorbing solvent such as water to recover (meth)acrylic acid in the gas in the form of a (meth)acrylic acid solution. This solution contains, in addition to (meth)acrylic acid, various impurities formed as by-products during the vapor phase catalytic oxidation, such as acetic acid, maleic acid, acrolein, furfural, benzaldehyde, acetone, etc. in the case of acrylic acid. Many methods have been proposed for recovering purified (meth)acrylic acid from such a (meth)acrylic acid solution. However, the principal ones are such that the absorbing solvent and a part of impurities are removed from the (meth)acrylic acid solution in a preliminary purification step to obtain a crude (meth)acrylic acid substantially comprising (meth)acrylic acid, dimers thereof and other heavy components, and then such a crude (meth)acrylic acid is purified in a purification step to obtain a product having a desired quality.
b. For example, in recent years, acrylic acid has found an increase in its demand as a starting material for e.g. a food additive or a highly water absorptive resin for e.g. paper diapers. In such applications, highly pure acrylic acid is required. Namely, if crude acrylic acid is used as a starting material for a polymer of acrylic acid without removing impurities, there will be a problem such as a delay in the reaction during the polymerization reaction, a decrease in the polymerization degree or coloring of the polymerized product.
Accordingly, industrially, purification of acrylic acid is carried out by distillation. However, it is not easy to remove by distillation impurities in crude acrylic acid obtained by vapor phase catalytic oxidation.
Heretofore, as a method for producing highly pure acrylic acid by separating and removing impurities from crude acrylic acid obtained by vapor phase catalytic oxidation, a method of carrying out distillation in the presence of a hydrazine, has, for example, been known (JP-A-49-30312, JP-B-58-37290, etc.). However, such a method is primarily intended to remove an aldehyde in the crude acrylic acid, whereby removal of maleic acid and/or maleic anhydride (these will be together hereinafter referred to as “maleic acids”) tends to be inadequate.
Further, JP-A-7-330659 discloses a method of carrying out distillation in the co-presence of hydrazine and ammonia. This method is effective for removal of maleic acids, but has a problem such that the added ammonia will be distilled from the top, such being not suitable for the production of highly pure acrylic acid. Further, this application discloses batch treatment only, and discloses nothing about a method for continuously obtaining highly pure acrylic acid on a commercial scale.
Accordingly, it has been considered that with these techniques, it is not easy to continuously produce high purity acrylic acid by sufficiently removing impurities containing maleic acids from the crude acrylic acid.
On the other hand, JP-A-2001-316326 discloses a method for continuously producing high purity acrylic acid by preventing sludge formation in the distillation column, wherein crude acrylic acid having a concentration of maleic acids of at most 2000 ppm, is used as a starting material for high purity acrylic acid. However, in order to reduce the concentration of maleic acids in the starting material crude acrylic acid, it is necessary to remove maleic acids in the step of obtaining the crude acrylic acid, and such does not provide a substantial solution to the problem. Further, in the process for producing acrylic acid, for example, in a step of recovering acrylic acid from the bottom residue of the acrylic acid distillation column, maleic acids, will be distilled in acrylic acid, and consequently, accumulation of maleic acids will take place within the acrylic acid production process, and accordingly, from the industrial view point, it is desired to develop an economically excellent method whereby crude acrylic acid containing at least 2000 ppm of maleic acids can be used as the starting material, and yet, high purity acrylic acid can be continuously produced constantly.
c. On the other hand, as a purification method for (meth)acrylic acid obtained by vapor phase catalytic oxidation of propylene or isobutylene, a distillation method is common, but (meth)acrylic acid is extremely susceptible to polymerization, and its handling was problematic.
d. As one of distillation apparatus, a vertical thin film evaporator is known which comprises an evaporator main body with its principal portion being cylindrical, which has a heating means on its exterior surface, a liquid inlet and a vapor outlet at its upper portion and a residue discharge port at its lower portion, a rotary shaft set in the main body, and stirring vanes attached to the shaft and being movable in a peripheral direction along the inner wall surface of the evaporator main body. With this thin film evaporator, the interior surface corresponding to the exterior surface on which the heating means is provided, is a heat transfer surface, and a liquid to be treated, which is supplied from the upper liquid inlet will be pressed and spread in a film form on the cylindrical inner wall surface by the rotating stirring vanes, and in the process where this liquid film falls by gravity, low boiling point components in the liquid to be treated are permitted to evaporate by the heat supplied from the heating means. This apparatus is capable of evaporating low boiling point components in the liquid to be treated, in a short time, and thus, it is suitable for treating a liquid containing a substance sensitive to heat, such as a readily-polymerizable compound. Further, the treated liquid is forcibly stirred by the rotating stirring vanes, whereby the liquid in contact with the heat transfer surface is always renewed by a fresh liquid, whereby there is a merit such that local overheating of the liquid to be treated can be prevented, and baking or scaling of the liquid tends to scarcely occur.
Many methods are available for attaching stirring vanes to the rotary shaft. For example, in a movable vane system, the stirring vanes are attached to the rotary shaft via fulcrums or springs, so that they can be moved in a circumferential direction about the rotary shaft, whereby by rotation of the rotary shaft, they rotate while contacting with the cylindrical inner wall surface or while maintaining a slight distance therefrom, by a centrifugal force.
e. On the other hand, heretofore, it has been common to employ a method for producing an acrylic ester by an esterification reaction of acrylic acid with an alcohol. As the acrylic acid to be used, one obtained by a vapor phase oxidation reaction of propylene, followed by dehydration, removal of low boiling point impurities and further purification treatment for removal of e.g. high boiling point impurities, may be used. However, it has been regarded advantageous to employ one not subjected to treatment for removal of high boiling point impurities, since purification costs of acrylic acid can thereby be made low (JP-A-9-157213, JP-A-10-237012, JP-A-10-306052, JP-A-2001-213839).
However, if acrylic acid containing high boiling point impurities, is used as the starting material, there have been problems such that undesirable polymerization reactions or side reactions are likely to take place, thus leading to clogging of apparatus such as pipes by polymerized products, deterioration of unit consumption of main materials such as acrylic acid and an alcohol, and a decrease in the quality of the product.
f. The acrylic acid-containing gas obtained by vapor phase oxidation will then be contacted with water in a collection column to obtain an aqueous acrylic acid solution, and an azeotropic agent is added to this aqueous acrylic acid solution, whereupon in an azeotropic agent dehydration distillation column, an azeotropic mixture comprising water and the azeotropic agent, is distilled, while crude acrylic acid containing acetic acid is recovered from the bottom of the column. Then, this crude acrylic acid is subjected to a distillation column for separating low boiling point components thereby to separate low boiling point impurities such as acetic acid, and further, high boiling point impurities are removed in a distillation column for separating high boiling components, to obtain purified acrylic acid. Further, there may be a case where acrylic acid is collected by contacting it with a high boiling point solvent in a collection column.
Acrylic acid thus produced, may be used as a starting material for various acrylic esters. In recent years, its demand as a starting material for a highly water absorptive resin has increased. Such acrylic acid as a starting material for a highly water absorptive resin is required to be acrylic acid purified to a high purity, and especially, aldehydes are required to be highly removed, since they tend to hinder a polymerization reaction or they tend to color the product polymer.
Heretofore, as a method for removing aldehydes simply and efficiently from purified acrylic acid, a method is known wherein aldehydes are converted to heavy substances by means of an aldehyde-removing agent of e.g. an amine system including a hydrazine system or an amino acid system (JP-A-49-30312, JP-A-49-95920, JP-B-50-14, JP-A-10-204024), a hydrogen sulfite system (JP-A-7-330672), a mercaptan system (JP-A-60-6635) or a combined system of a hydrazine system and a dithiocarbamate system (JP-A-7-228548), followed by distillation in a distillation column for purification, to obtain high purity acrylic acid from the top of the distillation column for purification.
The bottom fraction containing high boiling point compounds formed at the time of this distillation, contains, together with reaction products of aldehydes with an aldehyde-removing agent, a polymerization inhibitor such as hydroquinone added at the time of the distillation, and further high boiling point substances formed during the distillation, such as many heavy substances, such as an acrylic acid dimer (β-acryloxypropionic acid) or oligomers being Michael adducts of acrylic acid, polymers, etc.
Heretofore, such bottom fraction was disposed, or recovered for the production process for acrylic acid. This bottom fraction contains acrylic acid dimer, etc. being Michael adducts, and if it is recovered for the production process for acrylic acid, it is considered preferred to treat it in a thermal decomposition treatment step before recovery in the purification step for acrylic acid (JP-A-2001-213839).
g. It is an object of the present invention to solve the above-mentioned conventional problems and to provide a method for producing high purity (meth)acrylic acid by sufficiently removing impurities such as aldehydes, ketones, dicarboxylic acids such as maleic acids from crude (meth)acrylic acid obtained by a vapor phase catalytic oxidation method, which is an economically excellent method for producing (meth)acrylic acid, whereby continuous operation for a long period of time is possible while suppressing formation of sludge in the distillation column.
h. Further, as mentioned above, a high purity (meth)acrylic acid product is required as the starting material for a water absorptive resin such as paper diapers. The reason is that if an impurity, particularly furfural, is contained in the above-mentioned (meth)acrylic acid obtained by vapor phase catalytic oxidation, there will be a problem such as delay in the reaction, deterioration of the polymerization degree, coloration of the polymerized product, etc. at the time of the polymerization reaction for a water absorptive resin. Therefore, industrially, purification of (meth)acrylic acid is carried out by distillation or crystallization. Crystallization usually requires a large initial investment, and from the economical viewpoint, a method by distillation is employed in many cases, but it is difficult to remove the above impurity, particularly furfural, by usual distillation.
In order to solve this problem, a method has been proposed wherein a hydrazine compound is added at the time of purification of (meth)acrylic acid. This method is effective from the viewpoint of removal of the above-mentioned impurity, but has had a problem that it causes polymerization of (meth)acrylic acid during the rectification.
Formation of a polymer causes clogging in the distillation column, whereby the performance of the distillation column decreases, or it will be required to stop the operation. Accordingly, a method for suppressing formation of such a polymer, is desired.
JP-A-7-228548 proposes to suppress the formation by adding copper dithiocarbamate. In an operation for a short time, the effect of this method is confirmed, but in a continuous operation for a long time for a usual industrial operation, the effect has been still inadequate.
i. Further, in the purification of (meth)acrylic acid or its ester by distillation, if it is attempted to recover (meth)acrylic acid or its ester by means of a thin film evaporator from a heavy component containing (meth)acrylic acid or its ester discharged from the bottom of the distillation column, there is a problem such that clogging frequently occurs at a liquid-withdrawal tube or at an outlet portion of a liquid collection part of the thin film evaporator.
The present inventors have sought to find out the causes and as a result, have found them to be such that the liquid flowing down on the inner wall surface of the thin film evaporator will polymerize on a lower inner wall surface rather than at the lower end of the stirring vanes, and as the liquid introduced into the same film evaporator is concentrated, a polymerization inhibitor preliminarily added for (meth)acrylic acid or its ester, will precipitate.
Namely, in the thin film evaporator, stirring vanes are disposed to stir the liquid film on the heat transfer surface where evaporation takes place, and no stirring vanes are disposed at the inner wall surface portion below the heat transfer surface, particularly at the inverted corn-shaped liquid collection portion following the cylindrical portion, or at the funnel-shaped liquid collection portion being a combination of the inverted corn-shape and a cylindrical shape. Accordingly, at such a portion, the liquid flowing down, has the majority of low boiling point components removed and thus essentially has bad fluidity, and besides, no stirring by stirring vanes takes place, whereby the liquid in contact with the inner wall surface tends to be hardly renewed by a fresh liquid. Consequently, the retention time of the liquid in contact with the inner wall surface tends to be abnormally long, and (meth)acrylic acid or its ester remaining in the liquid, tends to gradually polymerize to change the liquid to be heavy, whereby the fluidity of this liquid further decreases, and a polymer tends to accumulate on the inner wall surface. Further, as the liquid is concentrated, the polymerization inhibitor preliminarily added for (meth)acrylic acid or its ester tends to be precipitated. The accumulated polymer and precipitates not only hinder the flow of the liquid, but also clog the outlet portion of the liquid collection portion or the following liquid withdrawal tube, if they are peeled off from the inner wall surface. Accordingly, it is an object of the present invention to provide a thin film evaporator free from such clogging.
j. It is an object of the present invention to avoid conventional problems such as clogging of apparatus such as pipes by a polymer, deterioration of the unit consumption of the starting materials, deterioration of the quality of the product, etc. and to provide a method for producing an acrylic ester which is economically excellent and industrially advantageous.
k. Further, it is not desirable to obtain high purity acrylic acid from the top of the above-mentioned distillation column for purification while subjecting the bottom fraction of the distillation column for purification to thermal decomposition treatment to recover it for an acrylic acid purification step, because as the thermal decomposition treatment is carried out at a high temperature, many side-reactions or decomposition reactions will take place, whereby formation of undesirable by-products which cause to accelerate polymerization of acrylic acid, to contaminate an acrylic acid product or to present coloration to the product, or regeneration of aldehydes, takes place, and such compounds are likely to be recycled to the purification step of acrylic acid.
To avoid such problems, a method has also been proposed wherein distillation is carried out under such a distillation condition that the acrylic acid concentration in this bottom fraction will be sufficiently low, and the bottom fraction is subjected to disposal treatment. However, if the concentration of acrylic acid in the bottom fraction is lowered, the viscosity of the bottom fraction will increase, whereby precipitation of a polymer, etc. tends to readily take place, thus leading to a trouble of clogging at the withdrawal pipe. Accordingly, the lowering of the acrylic acid concentration is limited, whereby it has been impossible to avoid a loss of acrylic acid in an amount corresponding to the one disposed as contained in the bottom fraction.
It is an object of the present invention to solve the above described conventional problems and to provide a method for producing high purity (meth)acrylic acid, whereby a highly purified high purity (meth)acrylic acid is produced by simply and efficiently removing aldehydes contained in (meth)acrylic acid, and at the same time, a waste liquid other than high purity (meth)acrylic acid fraction formed by this treatment of aldehydes, is recovered industrially advantageously.